As I was based at Heathrow for some time, I will offer my opinions of this mecca of duty-free stores...

The duty-free liquor stores at Heathrow are wonderful - good prices and a *great* range for those of us who like the more obscure/upper-end stuff. They usually have something on offer, (a 'six-pack' of malt whiskies, a free smaller bottle of the better/older stuff, etc), so if you don't have something definite in mind, choosing the current special offer may be a good way to go.

As far as other shops at Heathrow go - you need to know your prices. If you truly are saving the 17.5% VAT, it's worth shopping there (Thomas Pink is a good example). The chain stores, in general, are relatively good - you will save money if they have standardised pricing policies - but individual (unaffiliated) merchants tend to be a rip-off. Some of the stores at Heathrow will ship merchandise to your home address if you live in the UK (and possibly abroad too) - this is a big bonus for regular UK-based travellers who want to buy, say, a new suitcase or other bulky item, but don't want to lug it about with them.

Most of the same rules of thumb apply at Gatwick.

One thing *never* to buy at airports is a camera/film/anything else that is obviously aimed at tourists. I've found that drug stores & stationery stores don't offer a particularly good deal either.

As far as other airports go - it can be a good idea to buy the local specialty in the duty free. However, it may be cheaper elsewhere - but you will probably have to leave the tourist centre/ask a local where to find it.

Pricing in US dollars is becoming common in developing countries/countries with unstable currencies. In this case you are almost certain to be ripped off.

As an example - when I was at Dar es Salaam I had some time to kill and about 4000 (£4/$6.50) tanzanian shillings that I couldn't reconvert, so I wandered into the one of little stores outside the airport entrance. I found some Zanzibari spices and postcards that I wanted, but their ticketed prices added up to a little more than I had (5/6000 shillings) The lady serving me took my money anyway - don't be fooled by price tags! - and I got everything I wanted.

After I went into the airport and passed through immigration, the same basket of spices was priced at US$15 - and there was no negotiating!